1. Field of the Invention
This invention relates to laser sensors for measuring faint vibrations or pressure waves at low frequencies on land or at sea.
2. Related Art
Microchip lasers have been used in vibration detection in ultrasound U.S. Pat. No. 5,353,262 entitl ed xe2x80x9cOptical Transducer and Method of Usexe2x80x9d by Yakymyshyn, et al. discloses an optical transducer, for an ultrasound system, which includes a signal laser that generates an optical signal the frequency of which varies in correspondence with acoustic energy incident on the transducer. An optical cavity in the signal laser is disposed such that incident acoustic energy causes compression and rarefaction of the optical cavity, and this displacement varies the optical frequency generated by the laser. A laser pump coupled to the lasing medium is adapted to apply selected levels of excitation energy appropriate to the generation and detection of acoustic pulses. The signal laser alternatively is adapted such that the refractive index of the optical cavity is varied in correspondence with the incident acoustic energy to modulate the optical frequency of the light generated by the signal laser. A piezoelectric device is disposed to receive the incident acoustic energy and generate a corresponding electrical signal that is applied to an electro-optic cell in the optical cavity, or alternatively, to conductors to generate an electric field across the lasing medium.
U.S. Pat. No. 5,602,800 entitled xe2x80x9cMethods for Ultrasonic/Vibration Detection Using Polarization Beating In A Microchip Laserxe2x80x9d and U.S. Pat. No. 5,636,181 entitled xe2x80x9cUltrasonic/Vibration Detection Using Polarization Beating In A Microchip Laserxe2x80x9d by Duggal disclose methods for ultrasonic/vibration detection include using a sensor consisting of a microchip laser or an array of microchip lasers constructed to oscillate at two different laser frequencies corresponding to two orthogonal polarizations is disclosed. The frequency difference between these two different frequencies is chosen to occur at frequencies within the bandwidth of an electrical (as opposed to optical) signal processing system. When the microchip laser or microchip laser array is placed in an acoustic field, its cavity length is modulated, which causes a frequency modulation of the frequency difference between the two modes. When the two laser output polarizations are mixed using a polarization scrambling device, such as a polarizer at about 45 degrees to the polarization axes and then detected with a photodiode, one for each microchip laser, the resulting electrical signal contains the FM modulated beat frequency between the two polarization modes. This beat frequency is then demodulated using an electrical signal processing system.
Sensors for these two laser sensor applications have traditionally used piezoelectric transducers or linear induction transducers to generate a voltage or current that is proportional to a pressure, velocity or acceleration at a particular location. The U.S. Navy recently deployed optical fiber interferometric sensors, which give excellent sensitivity at multiple sensor locations along a single, light-weight electromagnetic interference immune fiber cable. Military applications, however, have the luxury of demanding the highest performance with concomitant high system costs. While several companies are exploring the use of optical fiber sensor technologies such as Bragg gratings for down-hole oil and gas sensor applications, the cost and sensitivity of such systems may preclude their ability to displace the highly cost-effective linear induction sensors used today.
The following is an example of an existing geophone sensor (e.g. the Mark 2 produced by Mark Products, Houston, Tex.) which has the following properties:
Hydrophones have similar specifications with sensitivity of 0.1 Pa. These specifications represent a formidable challenge for optical sensors. In addition to these steady-state requirements, the sensors must function while tilted in the Earth""s gravitational field (which is sizable compared with the accelerations being detected), and the system must work over a wide range of temperatures.
Based on these requirements, optical sensor designers have avoided using active optical devices at the sensor pod, due among other things to the challenging design problems presented when temperature cycling the sensor pod. Bragg gratings have been a preferred approach by many investigators over the last five years. The Bragg grating is formed inside of an optical fiber. The fiber is stressed, which causes a shift in the optical frequency, which is reflected back from the grating. Such sensors can also measure strain and temperature. The long-term stability of Bragg gratings is still being explored, but has shown good performance up to 100xc2x0 C. operating temperatures. Thus there is a need for a microchip laser that overcomes the limitations of the known prior systems discussed above.
Prior art passive optical sensors for seismic and underwater applications have relied on intensity modulation of an optical carrier. In normal operation, the optical intensity, detected by the receiver electronics varies as the fiber is thermally cycled or mechanically perturbed. This spurious signal can be mistaken as a signal originating from the sensor element. This is a disadvantage of intensity modulated designs. For phase modulated systems, the phase difference between two optical carriers is measured. In prior art designs, the phase difference between the two carriers is modulated by the sensor element. Spurious signals imparted along the cabling modulate the phase and amplitude of both carriers. If this common mode modulation is identical for both carriers, then the differential phase modulation is immune to the spurious signals (the common mode rejection is high). In practice, common mode phase and amplitude modulation is difficult to achieve, and as a result a high common mode rejection at the receiver electronics is difficult to achieve.
The common mode rejection of a phase-modulated sensor system can be improved by increasing the phase modulation depth imparted by the sensor element. For time-varying signals and large phase modulation depths, this is equivalent to frequency modulation. In this case, the frequency modulation imparted by the sensor element is much larger than any frequency modulation created along the cabling. The FM signal detected at the receiver electronics is substantially unaffected by the small phase modulations occurring along the fiber. FM sensor designs are also essentially immune to amplitude modulation along the cabling, since the receiver electronics is only measuring a frequency shift. The current invention uses FM for signal transport from the sensor element to receiver electronics.
The present invention provides a microchip laser geophone to enable a cost effective, performance-competitive sensor technology with single fiber connections between sensor pods, low sensor weight and reduced size which represents a unique opportunity to provide a system that provides new capabilities in the oil and gas exploration industry.
An optical geophone for detecting seismic vibrational energy comprising a laser material which generates a lasing frequency signal, the frequency of which varies in accordance with vibrational energy incident upon the laser material, a light source occurring at an excitation frequency for providing excitation energy incident upon the laser material, so that the laser material emits light at the lasing frequency modulated by the vibrational energy and a fiber optic cable for transmitting the excitation frequency light to the laser material and receiving the frequency modulated lasing frequency for transmission to the receiver. The laser material comprises a first face and a second face, the first face being parallel to the second face. The first face is coated so that it reflects almost all (e.g. 99% reflectance) of the lasing frequency light and passes substantially all (e.g. 90% transmittance) of the excitation frequency light. The second face is coated so that the second face reflects substantially all (e.g. 99.9% reflectance) of the lasing frequency light and substantially all (e.g. 99.9% reflectance) of the excitation frequency light.
The optical geophone further comprises an acoustic amplifier for coupling vibrational energy to the laser material, wherein the vibrational energy is incident upon a side of the laser material orthogonal to the incidence of the excitation energy. The laser material comprises, for example, a solid monolith of single crystal lasing material such as Nd:YAG. The laser material is polished to precise tolerances on two opposing faces, wherein the first face is coated with a dielectric mirror which provides 99%-99.9% reflectance at the lasing wavelength of the laser material, and passes almost all of the optical pump wavelength used to excite the laser (for Nd:YAG, this is at 808 nm); and the second face is coated with a dielectric mirror that is highly reflective at both the pump wavelength and the lasing wavelength. The excitation light source comprises an optical beam at 808 nm, the Nd ions absorb this energy and fluoresce at 1064 nm whereby the reflective surfaces of face one and face two form an optical cavity that provides sufficient feedback to cause lasing at 1064 run. The output beam is coupled through the first face mirror and travels antiparallel to the direction of the excitation light source, wherein the excitation light source is supplied by an optical fiber, such that the lasing beam is being coupled back into the optical fiber, so that the system is essentially self-aligning. The laser material is capable of lasing on a plurality of longitudinal and transverse optical modes simultaneously, however, the laser crystal thickness is selected to be small enough so that the laser lases in a single mode.